This invention relates to rotor blades for rotorcraft such as helicopters and the like, and more particularly to an improved construction and control scheme for such rotor blades which permits a significant reduction in noise generated by the blades.
Conventional helicopters in descent flight conditions frequently generate an impulsive noise signature which is commonly referred to as blade-vortex interactions (BVI) noise or "blade slap". BVI noise is generated by blade tip vortices, which interact with the rotor blades. Unfortunately, it is typically within a frequency range which is highly important to human subjective response. Additionally, it is easily detected electronically at large distances, thus increasing the vulnerability of military rotorcraft. Consequently, a reduction in the BVI noise intensity and changes in the noise signature, using active and/or passive noise control techniques, is desirable to the rotorcraft industry, which is challenged by today's stringent military and civilian acoustic regulations.
There are two possible measures which may be taken to reduce BVI noise. Namely, the tip vortex strength may be weakened, and/or the separation distance between the blade and the tip vortex may be increased. The result of both measures is a decrease in the strength of the interaction between the rotor blade and the tip vortices. Existing devices which have been used for reducing BVI noise include Higher Harmonic blade pitch control (HHC), which seeks to change the blade tip vortex strength, and thus the local aerodynamic conditions, through blade pitch changes. Other control means concentrate primarily on reducing the strength of the tip vortex through blade tip geometric modifications. Typical examples are the use of leading and trailing edge sweep, the use of blade anhedral, and the use of a subwing concept. All of these examples, excluding HHC, may be classified as passive control techniques. An example of another active control technique would be the use of tip air mass injection, which again has the purpose of weakening the blade tip vortices.
Each of the prior art solutions to BVI noise has been at least partially unsuccessful, either because of ineffectiveness or because of the solution's detrimental side effects with respect to the flight characteristics and efficiency of the rotorcraft. For example, HHC methods change the aerodynamic conditions along the entire blade in order to reduce BVI noise, due to the change in blade pitch. Passive BVI noise control methods are not adaptable to changing BVI conditions throughout the flight regime, which are associated with changes in descent rate and forward flight speed. Additionally, most of the prior art solutions to the BVI problem are deployed at all times, whether or not needed, often degrading flight performance unnecessarily.